Prefilled drug containers allowing for self administration of drugs are becoming increasingly prevalent, as self administration has clear benefits in terms of cost to health care providers as well as improving patient convenience.
One type of prefilled drug container is an autoinjector, which includes an automatic delivery mechanism which, once activated, provides for automatic delivery of the drug and sometimes also automatic needle insertion. In autoinjectors, the delivery mechanism includes a stored energy source which, when released, drives the drug delivery mechanism and optionally the needle insertion. The stored energy source acts on a drive member, such as a plunger rod, which engages a plunger within the drug container, and drives the plunger within the drug container. In many autoinjector designs, the same plunger rod also drives the drug container through the autoinjector housing to perform a needle insertion operation.
Prior to use, the stored energy source must be retained in an energised condition. The plunger rod can be used to retain the stored energy source by engagement of the plunger rod with a retaining element on or coupled to the housing of the device. The plunger rod is then loaded between the stored energy source, typically a compressed spring, and the retaining element and is held in this condition for an indefinite period between assembly of the device and use of the device.
Typically autoinjector mechanisms are manufactured using plastic components when possible because of the ability to accurately mould small mechanical components out of plastics materials at low cost. However, a problem with plastics components, particularly for drug delivery devices containing drugs with a long shelf life, is that plastics materials tend to creep and have relatively low tensile and compressive strength compared to some metals and other materials. A drive member which is loaded by the force of a compressed spring may deform or even break over time, such that the proper functioning of the autoinjector is impaired or prevented.
The plunger rod is generally required to have a small cross-section along at least part of its shaft length, because it may need to fit inside the drug container or the energy source such as a spring). This limits the strength of the shaft, and exacerbates the risk of deformation or breakage described above, particularly if the shaft is made of plastic.
Clearly a metal shaft has advantages, but the cost of a metal plunger rod can potentially be much higher than a plastic one, particularly if sophisticated manufacturing processes such as milling and turning are used. In order to minimise cost some autoinjector manufacturers have used flat blanked plates of metal, profiled so that their edges provide operational surfaces for instance to engage with mechanisms to retain the stored energy source and enable it to be released when appropriate. The disadvantage of this approach is that typically the edge of such a pressed plate can have burrs and sharp edges which have poor friction characteristics and can cause the performance of the aforesaid mechanisms to vary unacceptably.
It is an object of the present invention to address this problem